1. Field of the Invention
This invention relates to thin glasses having high refractive index (nd), a layer composite assembly comprising these thin glasses, a method for the production of the thin glasses and their uses.
2. Description of Related Art
Glasses having refractive indices of above nd=1.5 up to nd=1.7 are quite known. However, in the field of technical glasses these values are achieved by the addition of high amounts of lead oxide which is highly questionable from an ecological point of view and also detrimental in the case of economical large-scale processes. Known classical optical glasses with optical positions in the higher refractive index region which are used for light and image guides and thus fulfil the requirements of the classical application fields (i.a. imaging, microscopy, medical technology, digital projection, photolithography, optical communications engineering, optic/illumination in the automotive sector) normally are manufactured as a bulk material due to the geometry of their products which are produced from them (lenses, prisms, fibers i.a.). So, standard formats of the manufacturing process of optical glasses are sections of bars from continuous bar production, fiber core glass rods as well as optical blocks. With respect to the smallest geometric dimension normally a thickness (sections of bars) or a diameter (fiber core glass rods) of 20 mm are minimum dimensions which are considered to make sense in an economic and applicative point of view, and desirable are thicknesses of equal to or higher than 40 mm and in the case of optical blocks these values start at about 150 mm.
Technical glasses (produced according to technical processes of hot forming) typically have refractive indices of about 1.50. Glasses having refractive indices of higher than 1.6 are normally not suitable for technical processes of hot forming, because mostly they have a “steep” viscosity curve (strong change of viscosity with temperature) and mostly a high tendency to crystallization. In the case of the production of bars the tendency to crystallization is not a problem, because the glass melts are cooled down in such a short time that no crystallization takes place. In this context the quick increase of the viscosity with decreasing temperature is in fact an advantage.
Exactly these properties of the classical optical glasses are different from the properties of the technical standard glasses, the physicochemical property profiles of which are specifically tailored for the technical parameters of the manufacturing aggregates of technical glasses, such as flat glasses, thin glasses and tubular glasses, which are significantly larger in comparison to the manufacturing aggregates of optical glasses.
Technical glasses normally have a “long” viscosity profile, which means that their viscosity does not vary very much with changing temperatures. This is the reason for the longer times of the respective single processes and for generally increased process temperatures, which in the case of large technical aggregates has a less marked negative influence onto the profitability. Furthermore, there are also significantly increased lifetimes of the materials in the aggregates due to the flow conditions and the size of the aggregate. This is a very critical point for glasses with a high tendency to crystallization. Long glasses are advantageous in continuous large aggregates, since these glasses can be processed in a greater temperature range. So it is not necessary that the method is adjusted to the fastest possible processing of the glass which is still hot.
In the case that it would be intended to produce classical optical materials using a technical standard process for the production of flat glass (e.g. drawing, overflow fusion, down draw, rolling), the chemical composition of the optical glasses has to be changed, normally reduced in its content of that components which impart the desired optical properties to the optical glasses. Such measures would for example be the reduction of the proportions of TiO2, ZrO2, Nb2O5, BaO, CaO, ZnO, SrO or La2O3. This indeed results in longer glasses with less susceptibility to crystallization, but also in a remarkable loss of refractive index and dispersion properties.
A further problem is that the flat/thin glass processes being favored at the moment due to economic reasons involve certain chemical requirements for the glasses to be processed which cannot be fulfilled by the classical optical glasses: For example, in a floating process no components which are susceptible for redox reactions are allowed to be present in the glass. Thus for example, it is not allowed to use optical standard components such as the oxides of lead, bismuth, tungsten as well as the classical polyvalent refining agents (arsenic), the actual effect of which exactly is the shift of the redox equilibrium.
So in total in a contradictory manner these two classical groups of materials, the optical and the technical glasses, are different with respect to their processability.
There are numerous uses for thin glasses having a high refractive index besides the classical fields of application. Of course, there is the possibility to produce such thin glasses by reworking a bar of optical glass. But it is obvious that the steps of cutting and polishing of such bar sections are extremely expensive and in addition stress the glass very strongly. Thus, very small thicknesses of glasses with large dimensions cannot be achieved. When thin glasses are mechanically polished, then the surface properties are not optimal.
WO 2012/055860 A2 relates to transparent layer composite assemblies comprising opto-technical hybrid glasses having refractive indices of higher than 1.6. But the hybrid glasses described there do not contain zinc oxide. The reason is that it has been assumed that zinc oxide would result in crystallization during the hot forming step. But indeed, zinc oxide in combination with an appropriate amount of barium oxide is able to effectively prevent crystallization and thus is able to allow an economic production.
GB 2,447,637 B relates to an OLED layer composite assembly which may be used for illumination or display purposes. But in this case a substrate glass having a refractive index of only about 1.5 is used. The disadvantages associated therewith have to be weakened with an antireflection layer.
US 2012/0114904 A1 relates to flat glass containing iron oxide which may be used in OLEDs. In this glass the special ratio of BaO to ZnO is not fulfilled, because the glasses contain much more BaO than ZnO. Due to the different composition these glasses in comparison to the thin glasses according to the present invention have much higher melting temperatures and also hot processing temperatures. As a consequence the respective melts significantly stronger attack the refractory material used. In addition, the absence of enclosures in and the geometric uniformity of the final products are compromised.
US 2012/194064 A1 describes a diffusion layer for OLEDs. The glass used there contains much Bi2O3 and less SiO2 and BaO. The same applies to US 2011/287264 A1.
Especially for the use as a substrate or superstrate in an OLED or a photovoltaic module it is important that no or only less total reflection occurs between a flat glass and an adjacent layer. The refractive index of the glass used should be as high as possible, because in many applications in layer composite assemblies the glass is adjacent to a layer having high refractive index, such as for example ITO in OLEDs. When the light created in the OLED exits, then the light from the ITO layer has to enter into the superstrate made of glass. The higher the difference of the refractive index between the ITO layer and the glass, the more distinct is the total reflection at the interface. Thus, here economically produced thin glasses having high refractive indices can advantageously be used.